Various abbreviations that appear in the specification and/or in the drawing figures are defined as below:
BS Base Station
CN Core Network
C-RNTI Cell Radio Network Temporary Identity
LTE Long Term Evolution
NB Node B
NCC Next Hop Chaining Counter
NH Next Hop Chain
eNB evolved Node B
Identity ID
HO Handover
KDF Key Derivation Function
PCI Physical Cell Identifier
PRACH Physical Random Access Channel
RRC Radio Resource Control
RF Radio Frequency
RSRP Reference Signal Received Power
RSRQ Reference Signal Received Quality
UE User Equipment
The Rel-12 Study Item named as Small Cell Enhancement for higher layers in RAN2 has been discussed in 3GPP. In this Study Item, it is proposed to investigate solutions in regards to improving the mobility robustness, reducing signaling overhead towards the core network, and bettering inter-node UE context transfer procedure. In addition to these, small cell related mobility procedures will also be studied in this Study Item. In one of the mobility topics, a method called fast X2 HO is proposed.
The principle behind the fast X2 HO is that a target BS or eNB reserves a certain amount of resources for the fast X2 HO and indicates a predefined PRACH and associated C-RNTI to a source eNB. When a UE sends a measurement report to the source eNB and the source eNB ascertains there is a reserved channel for the fast X2 HO, it may indicate the predefined PRACH and associated C-RNTI to the UE via an RRC message, such as an RRCConnectionReconfiguration message. After that, the UE can set up an RRC connection with the target eNB directly using the predefined PRACH and the associated C-RNTI without a network HO preparation procedure. Therefore, the legacy HO signaling is omitted between the source eNB and the target eNB.
In the legacy X2 HO, the source eNB will derive the key KeNB* and send the pair (KeNB*, NCC) to the target eNB during the HO preparation. The target eNB will include the NCC into the HO command and send it to the UE, which is transparent to the source eNB. The UE will derive the same key KeNB*. In this manner, security communication can be established between the UE and the target eNB. More information regarding key derivations during HO can be found in section 7.2.8.4 of the 3GPP TS 33.401 V12.5.0 (2012-09), which is incorporated herein by reference in its entirety.
However, in the fast X2 HO, the target eNB and the UE will not have the correct cryptography keys for security communication since the X2 interface signaling during the HO preparation is omitted. Due to this, the fast X2 HO is not applicable at least from the perspective of security communications.